It is already known that certain 3-alkyl-1-cyanophenyl-uracils have herbicidal properties (cf. EP-A 0 473 551). Thus, for example, isopropyl 2-cyano-5-[3,6-dihydro-2,6-dioxo-3-methyl-4-trifluoromethyl-1(2H)-pyrimidi nyl]-4-fluoro-benzoate can be used for controlling weeds. However, at low application rates, the efficacy of this substance is not always satisfactory.